This disclosure relates to a porous carbon-carbon composite suitable for use as a substrate in fuel cells, for example.
Some types of fuel cells, such as proton exchange membrane and phosphoric acid fuel cells (PEMFC and PAFC), use porous carbon-carbon composites as electrode substrates, which are also referred to as gas diffusion layers. One example fuel cell substrate and manufacturing process is shown in U.S. Pat. No. 4,851,304.
One typical method of making a substrate includes: (1) forming a non-woven felt from a chopped carbon fiber and a temporary binder by a wet-lay paper making process, (2) impregnating or pre-pregging the felt with a phenolic resin dissolved in a solvent followed by solvent removal without curing the resin, (3) pressing one or more layers of felt to a controlled thickness at a temperature sufficient to cure the resin, (4) heat treating the felt in an inert atmosphere to between 750-1000° C. to convert the phenolic resin to carbon, and (5) heat treating the felt in an inert atmosphere to between 2000-3000° C. to improve thermal and electrical conductivities and to improve corrosion resistance. The art as illustrated by U.S. Pat. No. 4,851,304 is incomplete because it does not teach how to produce substrates with a uniform porosity, bulk density, and thickness in a high volume heat treating operation.
The porous carbon-carbon composites used in fuel cells typically have a porosity of 70-75%, which corresponds to a bulk density of 0.48-0.58 g/mL for an example substrate. It is desirable to control the porosity within a tight range because it affects the properties of the substrate that, in turn, influence the performance of the fuel cell. The thickness of these substrates ranges from 0.12-2.00 mm, but thicknesses in the range of 0.12-0.50 mm are more typical. These substrates typically have a planform size of 50-100 cm×50-100 cm. The 2000-3000° C. heat treating step, frequently referred to as graphitization, is done in known induction or Acheson type furnaces in an inert atmosphere. A typical furnace load may contain a stack of approximately 2000 substrates and is about 72-120 inches (183-305 cm) tall.
The thickness of each substrate decreases by about 33% during heat treat due to pyrolysis of the thermoset resin. There is a tendency for the substrates to warp as a result of this shrinkage. Spacer plates are placed between groups of 50-200 substrates in the heat treat stack to maintain the flatness of the substrates as they shrink during heat treat.
Example prior art heat treat assemblies 11 are shown in FIGS. 1 and 2. The arrangement illustrated in FIG. 1 depicts a heat treat assembly of a first generation substrate having a planform dimension D3. As the fuel cell was redesigned, a smaller substrate having a planform d3 was developed. However, the reusable tooling employed in the heat treat assembly 11 has not been changed as the substrates became smaller since there was no apparent need and due to the large expense of manufacturing new tooling for the heat treat assemblies.
It has been found that the bulk density of the heat treated substrate varies with its position within the heat-treat stack and more specifically with the local pressure within the heat treat stack. One skilled in the art can calculate the local pressure at any point in the stack by summing the weight of the substrates and tooling above the point and dividing it by the area of the substrates. FIG. 4 shows substrate density versus position within the furnace for the configurations shown in FIGS. 2. The relevant tooling in this instance consisted of ½″×33″×33″ graphite spacer plates placed between each group of 50 substrates. There was also a 48″ diameter lifting fixture and a 33″×33′×4″ base plate in the center of the furnace load. The pressure variation from the top to the bottom of this particular stack was analyzed and is shown in FIG. 5 as a graph of pressure versus position in the furnace. The sharp discontinuity in the center is due to the lifting fixture and base plate. The over-all pressure range is small; but has a significant influence on the porosity and bulk density of this porous carbon-carbon composite. The average pressure is 2.8 psi with a range of +/−2.3 psi or +/−82% from the top to the bottom. Substrates on the bottom of the heat treat assembly are most dense and those on top are the least dense. This is particularly true of arrangements such as those shown in FIG. 2. This results in a low process yield with a significant number of parts being unacceptable because they do not meet the density specification. There is a need for a heat treat tooling configuration that minimizes the pressure variation between the top and bottom of the heat-treat stack.